This invention pertains to coupling networks, and more particularly, is concerned with networks for coupling transmitters to antennas.
The problem of coupling power into a load with a high ratio of reactance to resistance (high Q) has been present for many years and many solutions have been proposed. For a given load the bandwidth and degree of match are related, and that the wider the bandwidth, the poorer the match within that bandwidth. The use of multi-element ladder networks to provide a suitable match over the desired bandwidth rapidly becomes complex with an extreme degree of difficulty in adjusting such networks. The losses increase as the number of components due to the inherent loss in each component. In addition, where high power is present, the component costs can be quite high so any benefits are quickly offset by the cost of such a network. Frequency change can also be difficult and a time consuming process.
Other arrangements, such as fast switching circuits to tune a matching network to follow the instantaneous frequency of the source have had only limited success. The problem with this arrangement is the lack of a suitable switching element that is fast enough and can handle high power with adequate linearity and high reliability. The linearity problem can manifest itself in spurious signals generated in the switching circuitry and leading to interference. Other methods of providing adequate bandwidth are based on resistive loading. Some times lumped resistance with a deliberate loss in efficiency is added. Alternatively, inherent losses in coupling network components provide resistance. The low efficiency of vacuum tube transmitters contributed to a significant source resistance which has a broadening effect on the system frequency response. Solid state switching mode designs, however, have a much higher efficiency with a corresponding lower internal source resistance. This reduces the bandwidth broadening effect of the source. To maintain this high efficiency of the modern switching mode sources, a reactive network should be used. Adding series or shunt resistance to improve the bandwidth of the load is not a practical solution at the high power levels required for effective communications in the lower portions of the radio spectrum; i.e., LF and lower.
There exists a need to provide a coupling network for coupling a signal at a frequency from a voltage source to a load, where the voltage source has a source resistance, the load has a load resistance and a load reactance, and the ratio of the load reactance to the load resistance greater than 100.
An object of the invention is to provide a means to couple efficiently a source of AC energy to a reactive load with a natural bandwidth much less than the distribution of energy in the source, such as coupling of a solid state, high power, VLF (very low frequency) transmitter to a small (in terms of wavelength) antenna structure. The antenna has an electrical Q that is very high, greater than 100, and a resulting natural bandwidth that i s much less than the bandwidth of the modulated signal.
Another object of this invention is a simple but unique network that efficiently couples the source (transmitter) to the reactive load (antenna). The coupling network is applicable to any such circumstance that may require the efficient coupling of a source to a highly reactive load.